1. Field of the Invention
The present invention relates to an exposure control apparatus used in a vehicle-mounted camera for taking an image ahead of a vehicle.
2. Description of Related Art
There have been proposed various techniques for performing an image processing on images ahead of a vehicle taken by a vehicle-mounted camera to recognize a white line painted on a road or three-dimensional object such as a preceding vehicle, to thereby issue a warning, or perform vehicle running control based on the recognition results. To increase the accuracy of recognition of a white line or a three-dimensional object, it is a matter of great importance to properly perform exposure control of the vehicle-mounted camera in accordance with brightness of an area ahead of the vehicle. For example, as described in Japanese Patent Application Laid-open No. 2009-157085, it is known to set a part of an image taken by a vehicle-mounted camera as a recognition area for recognizing an object to be recognized, and control exposure of the vehicle-mounted camera in accordance with brightness of the recognition area.
However, since the brightness of the road surface ahead of the vehicle varies depending on various factors such as the light of the headlight of an oncoming vehicle, the shadow of the entrance of a tunnel, and the light at the exit of a tunnel, it may occur that it is impossible to perform the exposure control well following the variation of the brightness. To deal with this problem, there is known a method that reduces the exposure of a vehicle-mounted camera when it is detected that the signals of an image taken by the camera are saturated, as described, for example, in Japanese Patent Application Laid-open No. 2007-28279.
However, since this method adjusts the exposure after detecting saturation of the signals of a taken image, it has a problem in that the exposure control cannot be performed well following variation of the brightness, and accordingly the recognition accuracy is lowered immediately after the brightness has changed greatly.